Heterogeneous Network
The large uptake of mobile broadband has resulted in significantly increased traffic volumes that need to be handled by the networks, for example Wideband Code Division Multiple Access (WCDMA) and High Speed Packet Access (HSPA). Techniques allowing cellular operators to manage their network more efficiently are therefore of great importance. For example, a few such techniques by which it is possible to improve the downlink performance could involve 4-branch MIMO, multi-flow communication, and multi carrier deployment.
Since the spectral efficiency per link is approaching theoretical limits, a next step could be to improve the spectral efficiency per unit area. In other words, additional features for HSDPA need to be introduced to provide a uniform user experience anywhere inside a cell. This can be done by changing the topology of traditional networks. Currently, the 3rd Generation Partnership Project (3GPP) is working on this aspect by considering heterogeneous network deployments.
Deployment of low-power nodes (LPNs) is seen as a powerful tool to meet the ever-increasing demand for mobile broadband services. A LPN may correspond, for example, to a remote radio unit (RRU), a pico base station, or a micro base station allowing the network capacity to expand in a cost-efficient way. It should be noted that the power that is transmitted by these LPNs is relatively small compared to that of macro base stations, e.g. 2W as compared 20W for a typical macro base station.
A network consisting of traditional macro NodeBs and LPNs is referred to as a Heterogeneous Network (HetNet). Two examples of use-cases for heterogeneous network deployment that may be envisioned are coverage holes and capacity enhancement for localized traffic hotspots.
Deployed LPNs in a heterogeneous network are typically classified as either co-channel, where each LPN has its own cell identity or scrambling code, or combined cell, where the LPNs have the same cell identities as the macro cell. A co-channel heterogeneous network 10 deployment is illustrated in FIG. 1 showing a macro node 18 providing a macro cell 14. A number LPNs provide cells 16 located within the macro cell 14. Wireless devices 12 are located within the macro cell 14, and some of the wireless devices 12 are located at or within the cells 16.
The fact that the different transmit powers of Macro nodes and LPNs creates an inherent Uplink (UL)/Downlink (DL) imbalance region in which the UL quality is better at LPN while the DL quality is better from macro. A scenario involving an imbalance region 26 between the transmissions 22 of macro node 18 and the transmissions 24 of a LPN 20 is illustrated in FIG. 2. The imbalance region is on one side delimited by a border 28 at which the path loss is equal and an UL handover is optimal, and on the other side by a border 30 at which the DL received radio power is equal and a DL handover is optimal. A Soft Hand Over (SHO) region 32 is defined partly covering the imbalance region 26 on the LPN-side, and the overlap region 34 of the SHO region 32 and the imbalance region 26 is served by the macro node 18.
The robustness of control channel may be negatively affected when the User Equipment (UE) is in the imbalance region 26 and in SHO with both macro node(s) and LPN(s), and the macro node provides the serving cell. For example, DL channel quality indication (CQI) and Hybrid Automatic Repeat reQuest HARQ ACKnowledgement (ACK)/Negative-ACKnowledgement (NACK) transmitted on High Speed-Dedicated Physical Control Channel (HS-DPCCH) and the Happy bit with power allocation information transmitted on Enhanced-Dedicated Physical Control Channel (E-DPCCH) may not be received correctly. This is due to the UE transmission power is primarily controlled by the non-serving LPN(s) and the quality of DPCCH and other UL control channels is insufficient for the serving macro node. This will negatively affect both the UL and DL throughput.
On the other hand, UL TPC is carried on Fractional- Dedicated Physical Channel (F-DPCH) and sufficient F-DPCH quality needs to be guaranteed to ensure low enough TPC error. F-DPCH from the non-serving LPN(s) may consume high power due to the bad DL quality from non-serving LPN. Although this may not lead to evident average DL energy increase in LPN, the DL capacity can still be impacted as we have to reserve power for the control channel according to the peak power consumption. Therefore limiting the peak F-DPCH power consumption is also important. With certain type of traffic, e.g. Voice over IP (VoIP), the number of users in the system could be great, and the overall F-DPCH power consumption could be significant. In this case it is more important to limit the F-DPCH power consumption. F-DPCH can be gated or sent intermittently when DPCCH is inactive, but it has to be transmitted when DPCCH is present.
Uplink Inner Loop Power Control
All network nodes in the Active Set (AS) send TPC commands to the UE. The uplink Inner-Loop Power Control (ILPC) adjusts the UE transmit power in order to keep the received uplink Signal- to-Interference Ratio (SIR) at a given SIR target, SIRtarget. Upon reception of one or more TPC commands in a TPC command combining period, the UE derives a single TPC command, TPC_cmd, for each TPC command combining period in which a TPC command is known to be present, i.e. for each period DPCCH is transmitted.
There are two algorithms to derive TPC command. In the first algorithm (Alg. 1), the UE derives TPC_cmd in each slot, which can take a value of either 1 (indicating an increase transmission power) or −1 (indicating a decrease in transmission power). Standard procedure is that the UE will lower its transmission power if one or more TPC commands are DOWN. Only if all TPC commands are UP, it will increase transmission power. This ensures that the lowest possible power for the UE to be heard by at least one of the nodes in the AS is used.
In the second algorithm (Alg. 2), the UE processes the received TPC commands in a 5-slot cycle, and derives one TPC_cmd every 5-slot cycle. When not in SHO, TPC_cmd equals 1 if all 5 hard decisions within a set are “1” and equals −1 if all 5 hard decisions within a set are “−1”. Otherwise, TPC_cmd equals “0” (indicating a hold in transmission power). During SHO, a first temporary TPC (TPC_temp) is derived for each radio link set, as in a non-SHO case. The UE then derives a combined TPC_cmd, which is set to
            TPC_cmd      ⁢                          ⁢      is      ⁢                          ⁢      set      ⁢                          ⁢      to        ⁢                  -          1      ⁢                          ⁢      if      ⁢                          ⁢      any      ⁢                          ⁢      of      ⁢                          ⁢              TPC_temp        i            ⁢                          ⁢      equals        -    1    ,            TPC_cmd      ⁢                          ⁢      is      ⁢                          ⁢      set      ⁢                          ⁢      to      ⁢                          ⁢      1      ⁢                          ⁢      if      ⁢                          ⁢      1      ⁢              /            ⁢              N        ·                              ∑                          i              =              1                        N                    ⁢                      TPC_temp            i                                >    0.5    ,      and    ⁢                  ⁢    TPC_cmd    ⁢                  ⁢    is    ⁢                  ⁢    set    ⁢                  ⁢    to    ⁢                  ⁢    0    ⁢                  ⁢    in    ⁢                  ⁢    all    ⁢                  ⁢    other    ⁢                  ⁢          situations      .      
The second algorithm makes it possible to emulate a smaller step size.
Inner Loop Power Control Restriction
In inner loop power control restriction, the DPCCH is solely power controlled by the serving cell and all other uplink physical channels are set relative DPCCH according to legacy operation. This power control operation is achieved either by having the non-serving cells always issue TPC UP commands, or by having the UE ignore the TPC commands from non-serving cells via an HS-SCCH order. To limit the interference in the non-serving cell(s), the serving grant needs to be reduced via Enhanced-Absolute Grant Channel (E-AGCH) or Enhanced Relative Grant Channel (E-RGCH). By letting the serving cell control the DPCCH implies that the DPCCH SIR in the non-serving cells will increase significantly. To make use of the increased DPCCH SIR in the non-serving cells, the reference value setting can be set more aggressively. The aim is essentially to have roughly the same throughput before and after the decrease in serving grant.
ILPC restriction can be adopted when the UE is in SHO with cells including a serving macro and at least one non-serving LPN. It can guarantee UL control robustness towards the serving macro. The scheme also works for legacy UE when implemented by sending TPC UP commands from LPN(s).
TPC Discarding
The idea behind TPC discarding is to take the radio link quality into account when deriving TPC commands. More specifically, the UE discards the received TPC commands with too poor quality. The quality criteria can for example be TPC error probability or the SIR of F-DPCH channel which carries UL TPC, for example F-DPCH that carries UL TPC. This can avoid that unreliable TPC bits are processed is and misunderstood by the UE leading to degraded UL performance.
Problems with Existing Solutions
As pointed out above, limiting the (peak) F-DPCH power consumption is important to avoid negative impact on DL performance, especially when there are many users in the system. There are certain situations where the transmitted TPC bits are, in some sense, redundant or superfluous even though DPCCH is present. For example, in ILPC restriction, the TPC bits from the non-serving LPNs are just used to disable the ILPC from LPNs, i.e. they do not impact the final ILPC results. Also when ILPC restriction is not used and there is a large imbalance, the UEs in SHO are effectively power controlled by a LPN which has a significantly better UL. In this case the TPC from macro is redundant.
Redundant TPC transmissions can be avoided in several ways. The F-DPCH carrying the redundant TPC can be transmitted with very low power or not transmitted at all, and the UE can discard the corresponding TPC. The second algorithm (Alg. 2) described above can be adopted and the F-DPCH carrying the redundant TPC can be transmitted with very low power or simply not transmitted, in which case it is very likely that the redundant TPC over the 5 slots is perceived as “0” (indicating a hold in transmission power). The UE can also be informed to ignore the TPC via a new HS-SCCH order.
In this context, low power means the power with which F-DPCH cannot be reliably received. F-DPCH is power controlled and adopting a transmitted power that significantly lower (e.g. −10 to −20 dB, or even turn off) than the power given by power control will certainly lead to a failed reception
However, there are certain problems with the above solutions. TPC discarding is not specified in the 3GPP specifications. Thus, it is likely that some UEs do not implement this feature. Further, the second solution adopting the second algorithm (Alg. 2) may work even if the UEs do not implement TPC discard. However, this may meet a problem if the number of nodes transmitting redundant TPC is equal to or more than the number of nodes transmitting effective TPC. In this case, the sum of TPC_tempi will never be greater than 0.5, i.e. the UE cannot increase its transmission power. Moreover, the effective power control step size of the second algorithm (Alg. 2) is small (⅕ dB), which may not be enough. Further, anew HS-SCCH order is not backwards compatible.
The relative F-DPCH power consumption can be high when the DL quality is bad. This can for example happen for transmission from LPN when there is large imbalance region, as is explained above. The DL performance can be negatively affected as power has to be reserved for the control channel according to the peak power consumption.